Builder's bum

[Dudders123]

My 4 year old son can't keep his trousers up! I've tried everything from adjustable waists, pull cords, elastic but nothing works. They all sit half way down his bum and threaten to come down completely when he starts to run! His teachers have started mentioning it to me like I'm not dressing him properly I'm at my wits end with it!
He's tall so needs the length, slim but with a belly and no bum so nothing stays up. It's not as bad in summer when I can usually find some elasticated shorts in a much smaller size but with trousers it's hopeless. He can't wear a belt as he's bad enough with getting to the loo on time....
Anyone with the same problem and any recommendations of particular brands / styles please?
Thanks x

Mini trouser suspendor?

My 4 year old also needs the length but they are huge at the waistband. we use next and tighten the buttons on the inside as tight as they go and it definitely works for us. He also has a belt that we have taught him to open for toilet visits when he's at school but we help him at home.

Thanks. I've tried adjustable waistbands as tight as they can go... they seem fine when first on but within 5 minutes the builder's bum appears!
May have to try the belt idea and tell him to get to the loo well before he's desperate!